In digital tomosynthesis (DT), a 3-dimensional image of an object or anatomy is typically generated from a limited number of low-dose x-ray projections acquired from different angles. The x-ray source is typically moved in an arc around an object being imaged (such as a breast) while a series of projection images are captured with a detector including an array of pixels. The arc is along a scan direction that is aligned with rows of pixels within the pixel array. Data from the resultant projection images is then processed by a computer to create a 3-dimensional tomographic volume. In breast imaging, digital breast tomosynthesis (DBT) has been shown to improve sensitivity and specificity for cancer detection relative to traditional two-dimensional projection mammography. In chest imaging, DT has been shown to improve the sensitivity and specificity of lung nodule detection relative to traditional two-dimensional projection radiography. DT has also had value in musculoskeletal imaging and in non-destructive testing.
DT's oblique x-ray incidence shifts the image of features within an object in sub-pixel detector element increments along the scan direction with each projection angle. As a result of this property, DT is capable of “super-resolution”, a term which is used to denote sub-pixel resolution, i.e., resolution that is finer than the physical size of the detector elements. Although super-resolution is achievable over a broad range of positions along the scan direction (e.g., parallel to the chest wall side of the breast support in current DBT systems), it cannot be achieved over a broad range of positions perpendicular to the scan direction (e.g., the chest wall—nipple direction in current DBT systems). This is because, for example, in current DBT systems the translational shifts in the image between projections are minimal or non-existent in the posteroanterior (PA) direction.
Higher resolution images are useful in the accurate detection and diagnosis of cancer, bone fractures, and other fine details. In breast imaging, for example, the presence of lesions, such as microcalcifications, can indicate the early stage of breast cancer. The form and morphology of the microcalcifications are important factors in determining whether the microcalcifications are benign or malignant. Improved visibility and conspicuity of lesions help in the determination of the probability of malignancy. It is therefore desirable to determine and set acquisition parameters to optimize super-resolution.
In DT x-ray systems, there are two acquisition modes for x-ray tube motion: step and shoot motion (SSM) and continuous tube motion (CTM). For a rapid scan time, SSM systems are mechanically limited. CTM systems reduce the scan time but the movement of the x-ray tube during exposures produces focal spot blur which can result in image blurring. The effect of blurring can limit the spatial resolution and reduce the visibility of details.